1. Field of the Invention
The present invention relates to satellite data distribution systems and to communication service systems, and in particular, relates to a data receiving method and unit suitable for receiving computer data which are electronically distributed to individuals.
2. Description of the Related Art
The moving picture experts group phase 2 (MPEG-2) standard is an encoding method applied to current broadcasting, high definition television, audio visual apparatuses, etc. By implementing broadcast service using digital broadcast signals encoded using the MPEG-2 standard, the number of channels can be increased over that in conventional analog broadcasting, and the cost of each channel can be reduced. This enables services such as broadcasts of not only pictures and sound, but also of various types of data.
In mutichannel digital broadcasting, various data broadcasts utilize a satellite's high transfer speed (e.g., a maximum of 30 Mbps in the case of a 27-MHz transponder). The data broadcasts include, for example, a music service, a game service, a magazine information service, and an Internet (WWW) service.
When a data distribution service is broadcast by a communications satellite, it is necessary to provide a system in which only subscribers can receive the service. For example, for many uses such as when a data provider desires to transmit data only to subscribers, or each user at a receiving side desires to receive its unique secret information from the provider, a personal secrecy protection method is required.
When a high speed data transfer at a maximum of 30 Mbps from a communications satellite is considered, it is found that the receiving side must decode encoded data in real time and at a high speed.
Also, in the case in which a satellite broadcast receiver or a communication service receiver uses a receiver terminal to simultaneously receive a plurality of different secret data (a plurality of programs) which can be received only by the one receiver, a device is required which decodes the received data while retrieving and switching decoding keys which sequentially change in real time.
When a receiver terminal for use in the above data broadcast service is considered, it is important to use a single piece of hardware to efficiently perform functions required for receiving satellite data, functions for decoding received encoded data in real time, functions for switching functions required for decoding, etc. In this type of service which performs, at a high speed, functions of switching decoding keys required for decoding, it is possible that an incorrect decoding key may be set in a decoder of the terminal, depending on the method used for setting the decoding key. The setting of an incorrect decoding key causes incorrect decoding, making it impossible to reproduce the original data. Inspection of the received data or transfer of error data to a computer is not possible in the computer itself (because the computer may crash in the worst case). Accordingly, in order for the receiver terminal to have a way of decoding data in real time, it is preferable that the receiver terminal have a self-diagnosis function to determine whether decoding has been correctly performed. However, no data receiving system has such a function.